Coating and surface repair method

ABSTRACT

A coating and surface repair method is provided. The method includes positioning a high velocity air fuel (HVAF) gun proximate to a damaged hardened steel surface and forming a ceramic metallic coating on the damaged hardened steel surface by executing an HVAF thermal spray process using the HVAF gun.

FEDERAL RESEARCH STATEMENT

This invention was made with government support under W911 W6-06-2-0002 awarded by NASA. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to a coating and surface repair method and, more particularly, to a coating and surface repair method for shafts, bearings and gears having hardened steel surfaces exhibiting certain types of damages.

Aircraft, such as helicopters of varying configurations, include many components that contact one another at high speeds and at high temperatures. These components include shafts, bearings and bearing races and gears that register with complementary gears. The repeated contact between bearings and bearing races and between gears and other gears leads to damage to various surfaces of those features over time. This damage can be exacerbated by exposure and corrosion.

Statistical data obtained from overhaul shops reveals that thousands of the components noted above (i.e., expensive bearings and gears) are damaged and get thrown away every month due to corrosion induced pit formation on bearing journals and integral raceways among other factors. Repair of the components is difficult and may be complicated by the risk of moisture adsorption in oil, which is often hard to control. In particular, the repair of high hardness surfaces of the components demands an optimization among various conflicting requirements.

The requirements include, but are not limited to, the requirement that the repaired surfaces maintain high hardness, the requirement that deposited materials be thermally and galvanically compatible with substrate steel, the requirement that the adhesion of the small repair volumes be adequate and assessable, the requirement that heat induced by the repair processes cannot over-temper the substrate steel, the requirement that a heat affected zone (HAZ) at an interface of the repair be small and cannot occur within the first 0.008-0.010″ depth of the active load bearing surface of the component and the requirement that the fatigue and mechanical strength properties of the repaired surfaces be comparable to that of the original surfaces.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a coating and surface repair method is provided and includes positioning a high velocity air fuel (HVAF) gun proximate to a damaged hardened steel surface and forming a ceramic metallic coating on the damaged hardened steel surface by executing an HVAF thermal spray process using the HVAF gun.

In accordance with additional or alternative embodiments, the hardened steel surface includes carburized steel.

In accordance with additional or alternative embodiments, the hardened steel surface includes carburized steel, nitridized steel, borided steel, cyanided steel, carbo-nitrided steel or a combination thereof.

In accordance with additional or alternative embodiments, the damaged hardened steel surface is pitted.

In accordance with additional or alternative embodiments, the forming results in a repaired surface with fatigue and mechanical strength properties comparable to that of the original damaged hardened steel surface.

In accordance with additional or alternative embodiments, the damaged hardened steel surface has defects of sizes between 0.005-0.040 inches in diameter and depth. In accordance with additional or alternative embodiments, the executing of the thermal spray process includes maintaining combustion temperatures within a range of 1900-2300° C.

In accordance with additional or alternative embodiments, the ceramic metallic coating includes a Tungsten-Carbide (WC) material.

In accordance with additional or alternative embodiments, ceramic metallic coating includes a Tungsten-Carbide Cobalt (WC/Co) material.

In accordance with additional or alternative embodiments, the ceramic metallic coating includes a Tungsten-Carbide Cobalt-Chromium (WC/Co—Cr) material.

According to yet another aspect of the invention, a coating and surface repair method is provided and includes positioning a high velocity air fuel (HVAF) gun proximate to a carburized steel surface exhibiting pitting damage, executing an HVAF thermal spray process using the HVAF gun with respect to the carburized steel surface and forming a ceramic metallic coating including a Tungsten-Carbide (WC) material on the carburized steel surface as a result of the executing of the HVAF thermal spray process.

In accordance with additional or alternative embodiments, the forming results in a repaired surface with fatigue and mechanical strength properties comparable to that of the original carburized steel surface.

In accordance with additional or alternative embodiments, the damaged hardened steel surface has defects of sizes between 0.005-0.040 inches in diameter and depth. In accordance with additional or alternative embodiments, the executing of the thermal spray process includes maintaining combustion temperatures within a range of 1900-2300° C.

In accordance with additional or alternative embodiments, the ceramic metallic coating includes at least one of a Tungsten-Carbide Cobalt (WC/Co) material and a Tungsten-Carbide Cobalt-Chromium (WC/Co—Cr) material.

These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a flow diagram illustrating a surface repair method in accordance with embodiments;

FIG. 2 is a perspective view of an aircraft component to be repaired in accordance with embodiments;

FIG. 3 is a schematic illustrating of a system for conducting a repair method in accordance with embodiments;

FIG. 4 is an enlarged view of the encircled portion of the surface of the system of FIG. 3; and

FIG. 5 is an enlarged view of the portion of the surface of FIG. 4 in a repaired condition.

The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The description provided below relates to a method for the coating and repair of hardened steel (e.g., carburized steel, nitridized steel, borided steel, cyanided steel, carbo-nitrided steel or a combination thereof) shafts, gears and bearing surfaces through a high velocity air fuel (HVAF) thermal spray process using a ceramic metallic (CERMET) material as a coating so that damaged surfaces of the affected parts can be repaired. In particular, the method can repair a carburized stainless steel SS9310 surface with defects of sizes between at least about 0.005-0.040 inches, while maintaining a defect-free interfacial bond with no reduction in substrate hardness, low coating porosity, high coating and substrate hardness, acceptable microstructure and surface finish.

The method is inexpensive compared to D-Gun or high velocity oxy fuel (HVOF) processes and has a better resulting surface finish than alternative methods, a smaller repair area with a greater deposit depth than alternative methods and minimal hardness changes to interfacial heat affected zones (HAZ) due to relatively lower temperatures. The method also permits the formation of coating thicknesses of about 0.005-0.040 inches, which are thicker than what is otherwise possible with alternative methods, and also requires no additional expensive grinding processes (e.g., no need for final surface grinding or initial grit blasting) as with processes in which D-Gun coatings of WC/Co are formed.

With reference to FIGS. 1-5, a coating and surface repair method in accordance with embodiments is provided. The method includes positioning a high velocity air fuel (HVAF) gun 10 proximate to a damaged hardened steel surface 20 (operation 100), executing an HVAF thermal spray process using the HVAF gun 10 with respect to the damaged hardened steel surface 20 (operation 110) while maintaining combustion temperatures within a range of 1900-2300° C. (operation 115) and a standoff distance of at least 7 inches and thereby forming a ceramic metallic coating 30 on the damaged hardened steel surface 20 (operation 120). In accordance with embodiments, the ceramic metallic coating 30 may include at least one or more of a Tungsten-Carbide (WC) material, a Tungsten-Carbide Cobalt (WC/Co) material and a Tungsten-Carbide Cobalt-Chromium (WC/Co—Cr) material. The hardened steel surface 20 may include at least one or more of carburized steel (e.g., a carburized SS9310 coupon surface), borided steel, cyanided steel, nitridized steel or some combination thereof. The forming of operation 120 results in a repaired surface 40. This repaired surface 40 has fatigue and mechanical strength properties that are comparable to that of the original damaged hardened steel surface 20.

As shown in FIG. 2, the damaged hardened steel surface 20 may be a bearing journal surface 21 or an integral raceway 22 of a gear 200. As shown in FIG. 4, the damaged hardened steel surface 20 may exhibit pitted-type damage, such as corrosion induced pitting, in which the surface includes recesses or pits 23 that are recessed from a normal plane P thereof. The pits 23 may have a defect size of about 0.005-0.040 inches in depth and/or diameter. As shown in FIG. 5, the repaired surface 40 includes the ceramic metallic coating 30 in the pits 23.

As shown in FIG. 3, the HVAF gun 10 is at least initially positioned at a distance from the damaged hardened steel surface 20 and includes a body 11 having a rear side 110 and a forward side 111. The body 11 includes an outer hull 112 and an inner hull 113 that are separated from one another to define a flow path 114, a ceramic baffle 115 and Hydrogen (H₂) or Nitrogen (N2), powder and fuel nozzles 116. The outer hull 112 is formed to define an inlet 117 by which air enters the flow path 114 and thus travels toward the rear side 110. The inner hull 113 is formed to define an interior, which is divided by the baffle 115 into a mixing chamber 118 in which the air, Hydrogen, powder and fuel are mixed to form a combustible mixture and a combustion chamber 119 in which the mixture is combusted. The combustion chamber 119 is tapered at the forward side to form an outlet nozzle 120 through which the combustion products are exhausted toward the damaged hardened steel surface 20 as a supersonic jet.

The HVAF gun 10 sprays powders, which are heated and accelerated by combustion products of hot compressed air and fuel gas. The fuel gas can be propane, propylene or natural gas.

In an operation of the HVAF gun 10, a mixture of air and fuel gas flows into the combustion chamber 119 through orifices defined in the baffle 115, which may be formed of a ceramic insert. Initial ignition of the mixture by a spark plug results in heating of the baffle 115 above an auto-ignition temperature of the mixture. The heat of the baffle 115 continuously ignites the mixture providing stable combustion within a range of air-to-fuel ratios and gas pressures. Cooling of the HVAF gun 10 may be provided by air or water or other types of coolants.

In addition to the air/fuel mixture, spray powder is injected axially into the combustion chamber 119, whereby the spray powder is heated at a pressure of about 2-5 bars. As an additional tool for accurate heating control, a high heat conductivity gas (e.g., hydrogen) may be injected into a powder carrier gas such that the powder is propelled into the outlet nozzle 120 where it is accelerated to supersonic velocities (over 1200 ft/s) to impact a substrate and to thereby form the ceramic metallic coating 30.

In accordance with embodiments, air-fuel combustion temperatures are only slightly higher than the melting temperatures of most coating metals/alloys. Thus, HVAF allows for the heating of spray powder particles near or slightly above their melting point but still within the plastic state. Such heat transfer regimes prevent excessive overheating of the particle surface allowing for better particle deformation and coating upon deposition. Also, since sprayed particles travel through the bigger combustion chamber 119 prior to reaching the narrow outlet nozzle 120, flow velocities and particle temperatures can be controlled and tuned for targeted coating coality.

This in turn allows for deposition of the ceramic metallic coating 30 in an acceptable window, free of oxidation or decomposition.

In accordance with embodiments, the execution of the thermal spray process of operation 110 requires that certain parameters be tightly controlled so as to avoid further damage to the damaged hardened steel surface 20. Such parameters may include those of the following Table:

Grit blasting None None Hardware AK-07 NOZZLE: 5L, AK-07 NOZZLE: 5L, CHAMBER: 3HP CHAMBER: 3HP INJECTOR: 3 CONSOLE: 2 INJECTOR: 3 CONSOLE: 2 Sample # Sample # 979-1 973-3 974-1 974-3 Thickness initial, inches 0.28 0.28 0.28 0.28 Target coating thickness, 0.011 0.021 0.031 0.041 inches Coating material WC-10Co-4Cr WC-10Co-4Cr Sample # 948-1 948-1 Spray parameters Air, psi 87 88 Fuel, psi 74.6 75.6 Combustion Pressure, 65 65 psi H2 flow % 35 35 N2 flow % 30 30 Powder Amperit 558.059 Amperit 558.059 Particle size, mk 5-30 5-30 Lot# 0601100 0601100 Rpowder 15 (#3) 15 (#3) Part rpm (on 10 inch 300 300 DIA) Gun traverse, cm/min 30 30 Stand-off, inches 7 7 T init, F. 290 290 T max, F. 325 325 T interim., F. 280-300 280-300 Cooling Air pipe Air pipe Number of passes 8 29 15 23

The use of the HVAF thermal spray process of operation 110 actually leads to surprising, unexpected results in that the repaired surface 40 has fatigue and mechanical strength properties that are comparable to that of the original damaged hardened steel surface 20 in spite of the fact that the temperature range of HVAF processes are normally about 1900-2300° C. In particular, the repaired surface 40 has about HRC 60-64 hardness without affected interfacial surface hardness, and about 2-3 RA surface finish without the need for surface grinding or grit blasting, and a coating thickness of about 0.005-0.040 inches. These surprising, unexpected results are at least partially produced by the parameters under which the execution of the HVAF thermal spray process of operation 110 is conducted. For example, with respect to the temperature ranges listed in the Table provided above, it is noted that the temperature of the damaged hardened steel surface 20 is generally maintained below 400° F. in order to avoid over-tempering the damaged hardened steel surface 20 at the normal HVAF temperature ranges noted above.

In accordance with embodiments, the sub-400° F. temperatures may be achieved through some combination of time limiting of the execution of the HVAF thermal spray process of operation 110, a focusing of the HVAF thermal spray process of operation 110 on a particular area of the damaged hardened steel surface 20 and a cooling operation. In the focusing case, the HVAF gun 10 may be position relatively close to the damaged hardened steel surface 20 or the jet generated by the HVAF gun 10 may be passed through a focusing element such as a tube or focusing nozzle. In the cooling operation, a coolant (e.g., water or air) may be directed around the HAZ during or after the execution of the HVAF thermal spraying. Alternatively, a cooling jacket maybe provided around the HAZ that prevents the HAZ from spreading over a large area of the damaged hardened steel surface 20.

In accordance with embodiments, the coolant may be provided by a coolant source 50 as shown in FIG. 3. The coolant source 50 may be disposed proximate to (i.e., in front of or behind relative to the HVAF gun 10) the hardened steel surface 20 and the coolant may be provided at a predefined temperature and velocity and with a predefined heat transfer coefficient. The temperature of the powder and hence the temperature of the ceramic metallic coating 30, assuming no heat generation or storage, has a linear relationship with the coolant temperature and the predefined heat transfer coefficient. Having a fixed L (i.e., ceramic metallic coating width), k (i.e., heat conduction factor of powder) and hardened steel surface 20 temperature that may not be exceeded, a required powder temperature with respect to the coolant temperature and coolant convection coefficient can be determined. Moreover, by decreasing the coolant temperature and increasing the coolant convention coefficient (either by increasing the velocity of the flowing coolant and/or using a more effecting heat transfer fluid), virtually all of the heat from the coating/powder is transferred directly into the flowing coolant, while maintaining a constant or reduced temperature of the hardened steel surface 20. Thus, successful repair of a damaged hardened steel surface with a tungsten-cobalt ceramic metallic coating using HVAF is possible so that the repaired materials have mechanical, wear and fatigue properties that are comparable to the original material.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A coating and surface repair method, comprising: positioning a high velocity air fuel (HVAF) gun proximate to a damaged hardened steel surface; and forming a ceramic metallic coating on the damaged hardened steel surface by executing an HVAF thermal spray process using the HVAF gun.
 2. The coating and surface repair method according to claim 1, wherein the hardened steel surface comprises carburized steel.
 3. The coating and surface repair method according to claim 1, wherein the hardened steel surface comprises carburized steel, nitridized steel, borided steel, cyanided steel, carbo-nitrided steel or a combination thereof.
 4. The coating and surface repair method according to claim 1, wherein the damaged hardened steel surface is pitted.
 5. The coating and surface repair method according to claim 1, wherein the forming results in a repaired surface with fatigue and mechanical strength properties comparable to that of the original damaged hardened steel surface.
 6. The coating and surface repair method according to claim 1, wherein the damaged hardened steel surface has defects of sizes between 0.005-0.040 inches in diameter and depth.
 7. The coating and surface repair method according to claim 1, wherein the executing of the thermal spray process comprises maintaining combustion temperatures within a range of 1900-2300° C.
 8. The coating and surface repair method according to claim 1, wherein the ceramic metallic coating comprises a Tungsten-Carbide (WC) material.
 9. The coating and surface repair method according to claim 1, wherein the ceramic metallic coating comprises a Tungsten-Carbide Cobalt (WC/Co) material.
 10. The coating and surface repair method according to claim 1, wherein the ceramic metallic coating comprises a Tungsten-Carbide Cobalt-Chromium (WC/Co—Cr) material.
 11. A coating and surface repair method, comprising: positioning a high velocity air fuel (HVAF) gun proximate to a carburized steel surface exhibiting pitting damage; executing an HVAF thermal spray process using the HVAF gun with respect to the carburized steel surface; and forming a ceramic metallic coating comprising a Tungsten-Carbide (WC) material on the carburized steel surface as a result of the executing of the HVAF thermal spray process.
 12. The coating and surface repair method according to claim 11, wherein the forming results in a repaired surface with fatigue and mechanical strength properties comparable to that of the original carburized steel surface.
 13. The coating and surface repair method according to claim 11, wherein the damaged hardened steel surface has defects of sizes between 0.005-0.040 inches in diameter and depth.
 14. The coating and surface repair method according to claim 11, wherein the executing of the thermal spray process comprises maintaining combustion temperatures within a range of 1900-2300° C.
 15. The coating and surface repair method according to claim 11, wherein the ceramic metallic coating comprises at least one of a Tungsten-Carbide Cobalt (WC/Co) material and a Tungsten-Carbide Cobalt-Chromium (WC/Co—Cr) material. 